Detection of high-frequency electric oscillations



J. L. LAWSON 2,472,378

DETECTION OF HIGH-FREQUENCY ELECTRIC OSCILLATIONS June 7, 1949.

Filed Aug. 27, 1945 FREQUENCY I I I 1 1 AMPLIFIER INVENTOR 43 T 4,0 JAMES L. LAWSON TO LF. AMP.

ATTORNEY Patented June 7, 1949 UNITED STAT TENT OFFICE DETECTION OF HIGH-FREQUENCY ELECTRIC OSCILLATION S Navy Application August 27, 1943, Serial No. 500,266

8 Claims.

This invention relates to apparatus for detecting high-frequency radio signals, and especially to apparatus for mixing signal oscillations with locally generated oscillations in an electrical circuit associated with a rectifier for the purpose of producing signal oscillations of lower frequency which may then be amplified in a fixed frequency amplifier. such apparatus may be briefly and generically referred to as a heterodyne detector or heterodyne mixer. In particular, this invention relates to heterodyne detectors adapted for operation at frequencies of the order of 3000 megacycles per second in association with rectifiers of the crystal type.

It is an object of this invention to provide a heterodyne detector apparatus for ultra-highfrequency operation in which the signal frequency and intermediate frequency circuits are suitably isolated and filtered. It is another object of this invention to provide a heterodyne detector which takes advantage of the tuning capabilities of adjustable coaxial cylinder resonators. A further object of the invention is to provide a resonator type detector in which adjustments and variable couplings are provided at locations of low resonator current.

According to this invention, advantageous reduction of losses in the signal and the desired isolating and filtering of the output circuit of the heterodyne detector are accomplished by producing the tuned circuit of the heterodyne mixer in the form of a coaxial cylinder resonator,

The invention is illustrated in the annexed drawings to which reference is made for further description of the invention and in which:

Fig. 1 is a diagrammatic cross section of a type of cavity resonator;

Fig. 2 is an axial cross section of one form of heterodyne detector apparatus constructed according to the present invention;

Fig. 3 is a cross section at right angles to the plane of Fig. 2 on the line 33 of Fig. 2, and

Fig. 4 is an axial cross section of modified form of part of the apparatus shown in Fig. 2.

Fig. 1 shows diagrammatically, in axial cross section, a coaxial cylinder resonator of the general form employed in connection with the preswill have a natural period of electrical oscillation which is determined by its dimensions. In fact, several natural periods will exist, corresponding to different modes of electrical oscillations which may be entertained in the cavity. For the purpose of the invention, however, only the simplest mode, which is also the mode having the lowest natural frequency, is of importance. The lines of electric intensity corresponding to this mode of oscillation are shown on Fig. 1. It will be noted that for the greater part of the length of the central post 8 there exists an electric field which is substantially radial'with respect to the said central post.

\ There is also an essentially axial field between the end of the central post 8 and the central part of the end wall 6. On account of the concentration of intensity of the latter field, the resonator may readily be tuned by making the end wall 6 in such a manner that it is movable axially with respect to the cylindrical wall 5. In such an arrangement the boundary between the end wall 6 and the cylindrical wall 5 does not carry particularly heavy currents, so that more than an ordinarily good electrical contact is not required. The radial field is important because it provides convenience of coupling to the cavity which may be by a coaxial line entering through the middle of the cylindrical wall 5, with its outer conductor connected to the cylindrical wall 5 and its inner conductor brought out into a small condenser plate approaching the central post 8. Even near the extremity of the post 8 the circumferential field has a considerable radial component. Advantage is taken of these properties of coaxial cylinder resonators in the form of heterodyne mixer apparatus shown in Figs. 2 and 3.

The type of electrical resonator shown in Fig. 1 has been called a coaxial cylinder resonator because it is characterized by the two cylindrical conducting surfaces 5 and 8. Such a resonator may be regarded as a resonant section of coaxial conductor transmission line closed by a short circuit at one end and by a capacitance at the other end or it may be regarded as a special form of cavity type resonator.

The resonator which forms the tuned circuit of the heterodyne detector apparatus shown in Figs. 53 and 3 may readily be made of brass. It consists of a brass cylinder it open at one end and at the other end closed by a conducting floor (preferably integral with the cylindrical Wall) having a central threaded hole through which a central post structure l5, 16, ll may be secured. After assembly, the outer part l5 of the central structure is preferably soldered in place to improve electrical contact between it and the outer shell 10. A cylindrical plug [2 is fitted by means of screw threads into the open end of the cylinder It. Its position is. adjustable within the cylinder l and this adjustment serves for the purpose of tuning the resonator. A tightening ring I3 is threaded on to the outside of the cylinder In in order to preserve the adjustment of the cylindrical plug l2. The upper threaded part of thecylinder l0 may be axially slotted as shown at M for gripping the plug l2 in cooperation with the tightening ring I3. The central. post of the resonator comprises an outer conducting sheath i5 which is in direct electrical contact with the outer metal walls of the resonator cavity, and an inner central conductor I6 and insulation I! electrically insulating the conductor Hi from the sheath l5 and the other conducting parts associated with the conducting sheath IS. The inner extremity of the central. conductor: I6: is. drilled at lfia to provide a seat for a clip [8. The clip i8 is in good electrical contact'with the conductor I6, having been: soldered thereto or having been pressed into a close fit in the hole drilled in the conductor 16. The outer sheath l5 and the insulation H; are provided. with apertures which are in registry with the hole [6a drill-edin the conductor l6; and are of such size asto accept a rectifier element, suchas. a cartridge lBa. which may be ofv thesilicon. crystal. type; The smaller metallic end. of cartridge.vv l8a' is: in.- serted into clip 18, and the larger metallic end is positioned by a hole.- in; the. outer wall of resonator IEI, thereby: holding cartridge 18a in such alignment that it does not. come into direct contact with the sheath [5; A. tightening ring 19 is provided for maintaining: in place the central conductor IE. Thetightener. t9. acts in cooperation with an insulatingwasher 20. acting on a small circumferential flange on the conductor It. An adjustable recess Zlf isprovided in the outer wall ll! of the resonatoropposite the clip IS in order to accommodate the outer end of the crystal rectifier cartridge. A threaded plug 22 provides means for holding the crystal rectifier cartridge [8a in place and" for removing and replacing the said cartridge.

Signal voltage and local oscillator voltage are introduced intothe resonator and coupled therewith through coaxial conductor transmission lines 23 and 2 4 respectively; which, as shown in Fig. 3, are brought into the cavity resonator at points preferably spacedQU on-the circumference of the cylinder Ill-from the position occupied by the crystal rectifier cartridge. It'is desirable that these voltages be introduced at places separated from each other by substantial spacing in order to minimize direct coupling between the antenna and the localoscillator.

A tubular brass fitting 25 is provided with insulating spacers 26" and 21 and central conductor 28 to form a short section of the coaxial transmission line 23. One end of the conductor 23 is provided with a slotted clip 2 9 The corresponding end of the tubular fitting 25 is threaded so that a further length of coaxial transmission line may be threaded thereon thus connecting the outer conductor of such length of line with the fitting 25 and the inner conductor of the said length with the conductor-'28 through the clip 29. The other end" of the fitting 25, shown in Fig. 3 as. having a somewhat reduced outer diameter, is outwardly threaded for insertion into a threaded hole in the brass cylinder it), being secured in place by a lock nut 30. When the fitting 25 is so inserted, as shown in Fig. 3,

there is a good electrical contact between it and 5 the wall of the brass cylinder ill, but the inner conductor is insulated from the cylinder 8. The inner end of the conductor 28 then protrudes into the resonant cavity enclosed by the cylinder ID. This end of the conductor 28 is terminated by an enlargement 3| preferably in the form of a small disk, which is adapted to act as one plate of a condenser the other plate of which is the inner central post of the resonator. At the other side of the cavity a similar structure, heretofore identified as part of the transmission line 2%, is inserted, having an outer conductor 35, an insulated inner conductor 36 and a terminal disk 37. If desired, the portion of the central post It nearest to the disks 3| and 31 may be flattened in order to make the electric field of the coupling capacitances more uniform.

The coaxial transmission line 24 is connected to a local oscillator. Likewise, the transmission line 23 which includes. coaxial structure 25, 28 is coupled with an antenna system for the picking up of a signal voltage and introducing such signal voltage into the resonator of. the mixer apparatus. It will be found advantageous to couple the signal voltage with the resonator more closely than the local oscillator output is coupled there-- with- This is because the signalvoltage is usually the weaker voltage and because transfer of local oscillator output to the antenna is. undersired. The terminal disk 3, which is connected with the antenna is therefore preferably brought closer to the central post of the resonator than the terminal disk 3! which is connected with the local oscillator. It will be seen that in this structure the degree of couplin between the respective coaxiallines 23 and 24: and the resonator may be readily adjusted by varying the distance between the central post. and the respective terminal disks 3| and 37.

The oscillatory energy introduced by the coaxial line 23 (Fig. 3) into the resonator is adapted to excite the resonator in its fundamental mode of oscillation, as discussed. in connection with Fig. 1, since the coupling condenser field between the disk 3! and the central structure l5 lies substantially in the same direction as a substantial component of the field associated with oscillations of said mode, such oscillations having a considerable radial electric field component about the circumference of the central structure as described inconnection with Fig. l. The plug 12 (Fig. 2) is adjusted to bring the resonator into resonance with the signal oscillations.

The local oscillator frequency is adjusted to 60 differ from the signal frequency by the amount of a predetermined intermediate frequency to which the following I. F. amplifier is tuned. An I. F. frequency of 30 megacycles per second is preferred, although frequencies from to 75 mc./sec. have been found capable of practical use as intermediate frequencies for the heterodyne detection of sginals having a carrier frequency of the order of 3000 mc./sec. In the detection ofsuch signals it will thus be seen that the local oscillator frequency is not greatly different from the signal modulated carrier frequency in terms of relative magnitude. The local oscillator frequency may be either higher or lower than the signal frequency, but in practice the lower os- 75 cillator frequency is generally preferred because better performance of local oscillator tubes and structures is usually obtainable at lower fre-- quencies than at higher frequencies. The superimposition of the local oscillator oscillations upon the signal frequency oscillation will, in the wellknown fashion, produce a modulation of the amplitude of the resultant oscillation at the said intermediate frequency.

Because the insulation I1 is relatively thin, the central post i6 is capacitively coupled to the conducting sheath l5. Because the coupling capacity of such a structure is relatively small, the coupling, although considerable at the signal frequency, will be practically negligible at the intermediate frequency. In consequence, the resultant radio-frequency oscillations occurring in the resonant cavity will, by virtue of the coupling existing between the conducting sheath i and the conductor [6, impress a resultant radio-frequency voltage across the crystal rectifier which voltage is modulated in amplitude at an intermediate or beat frequency. If, now, a circuit 50 tuned to the intermediate frequency is connected through a suitable choke 5| connected between the conductor [6 and ground (some suitable point on the resonator structure being also grounded) the rectification occurring in the crystal rectifier element will cause an alternating intermediate frequency voltage to appear across this tuned circuit. As shown diagrammatically on Fig. 2, such a tuned circuit 53 is adapted for coupling to the input 52 of a fixed-tuned intermediate frequency amplifier.

An apparatus for straight detection (without heterodyne action) can be provided by the construction shown in Figs. 2 and 3, with or without the modifications shown in Fig. 4, by omitting the coupling probe 31 and the coaxial conductor line 24, or simply leaving those elements disconnected. The advantages of the coaxial resonator form of tuned circuit and of the radio-frel quency by-passing provided in the central post structure may then be realized in connection with a straight detection operation. In general, heterodyne detection is to be preferred because of the large amount of amplification possible in a fixed-tuned intermediate frequency amplifier.

Fig. 4 shows a modified form of center post for use in a heterodyne detector structure of the general configuration of that shown in Figs. 2 and In the construction of Fig. 4, instead of the single fixed central conductor 16 a composite inner conducting structure is provided which comprises a central conductor such as a wire a head 4! in which is set a clip 42 corresponding in purpose and function to the clip 18 of Fig. 2, and a cylindrical skirt 43. Both the skirt Q3 and the wire 44] are in good electrical contact with the head 4| and all three parts are insulated from the conducting sheath I5. In this case, the insulation which insulates the central conducting structure from the conducting sheath i5 is composed of a cap 45 and washer 46 and 31, the skirt 43 being insulated from the sheath for the greater part of its length by an air gap. Between the skirt 43 and the wire 40 there is inserted an insulating plug 48. I prefer to make the larger pieces of insulation for this apparatus out of a material possessing low losses at the extremely high frequencies at which the apparatus is designed to operate. Such a material polystyrene. Some of the other insulating elements, however, such as the cap 45 in Fig. 4 and the spacers 2B, 21, 38 and 36 in Fig. 3 may be satisfactorily prepared from high quality Bakelite or similar insulating materials.

An important advantage of the structure of Fig. 4 lies in the fact that the skirt 43 in association with the insulation 48 and the conductor whichv is coaxial with its acts as a radio-frequency choke to prevent energy of oscillations at signal frequency from proceeding along the conductor 40 I and through the intermediate frequency circuit. An additional choke is therefore not needed in the intermediate frequency circuit.

The inside length of the skirt 43 should be a quarter wave length (or some odd multiple thereof). The presence of the polystyrene between the skirt 43 and the wire 40, however, will reduce the length of the electrical quarter-wave length as compared to what a quarter-wave length would be if the insulation were air instead of polystyrene. In other words, the choke consisting of the skirt 43, the wire 40 and the head 4| is to be considered as a quarter-wave section of coaxial line with polystyrene insulation, the line being closed at one end. The wave length in such a line is shorter than in a similar line employing air insulation. In consequence, the length of the skirt 43 will often be approximately of the wave length in free space of the oscillations in question, the exact length depending in a well-known manner upon the dimensions of the wire 40 and the skirt 43 as well as upon the dielectric constant of the insulation 48.

In the form of apparatus shown in Fig. 4 the dimensions of the central post structure are preferably so provided that the electrical length of the line made up of the skirt 43 with its associated structure as inner conductor and the sheath l5 as outer conductor is approximately a quarter-wave length. Since in the type of construction shown in Fig. 4 the dielectric of the aforesaid line is chiefly air, such electrical quarter-wave length will have a greater physical length than the electrical quarter-wave length of the line on the inside of the skirt 43, which is filled with a dielectric such as polystyrene. In this manner, a very low radio-frequency impedance at the intended frequency of operation is obtained between the upper part of the sheath I 5 and the conducting structure associated with the clip 42. At the same time there is a minimum of radio-frequency potential between the Wire 40 where it emerges from the central post structure and the grounded structure of the resonator, the nearest part of which is the tightening member 49.

By means of the above-described compound structure of the central post of the resonator of the mixer circuit, it is possible to obtain adequate filtering to prevent loss of signal energy in the intermediate frequency circuit and also to achieve proper isolation of the local oscillator and antenna circuits. In addition, the advantages of the high degree of resonance obtained in the resonant cavity type of tuned circuit are put to good account and a simple tuning adjustment is provided for tuning the detector apparatus to the signal frequency. Furthermore, the signal and local oscillator oscillations are brought into the resonant circuit of the detector through devices that permit a fine adjustment of the degree of coupling. A further advantageous feature of this invention is that the crystal rectifier can be con veniently mounted within the resonator in such a way that the current path of the detected modulated signal from the detector to the following I. F. amplifier is short and direct.

In the apparatus of this invention screw threads and other necessities of adjustability with doubtful contact characteristics may be limited to parts of the resonator which do not carry high current (at or near voltage points, that is). Likewise the coupling to the feed lines is at a voltage point rather than at a current point. As pointed out in the discussion of Fig. l, in resonators of the type embodied in this invention, regions of high currents appear near the lower part of the annular cavity, whereas regions of high voltage and low currents appear near the upper part of the cavity near the top of the center post.

It will be noted that in the apparatus of Figs. 2, 3 and 4, the crystal rectifier element is located near the extremity of the axial center post or column, bridging the resonator at the axial level. Since crystal rectifiers do not normally exhibit a very high radio-frequency impedance, the location of the crystal in this position will result in the impedance of the resonator frequency being somewhat larger than that of the crystal. Such impedance mismatch, in spite of the consequent reduction of energy transfer, may, as I have pointed out in a copending application, Serial No. 491,988, filed June 23, 1943, and entitled, Overload protection of high frequency receivers, produce advantageous overload protection and is therefore desirable in many cases if not provided in an excessive degree. The impedance match or mismatch between the crystal and the resonator may be set for desired conditions in the design of the mixer in view of the impedance of the crystals to be used, the need for overload protection, the importance of weak signal sensitivity, and so on, by locating the position of the crystal at a suitable level with respect to the axis of the resonator. Locating the crystal near the ex tremity of the structure [5, I6 will tend to increase the degree of mismatch by placing it across a higher impedance portion of the resonator, while locating it nearer the bottom or closed end of the resonator will tend to reduce the degree of mismatch, except that care should in any event be taken not to locate the crystal so far from the end of the structure l5, that it is mismatched in the sense opposite to that above described, for in such case an overload will tend to improve the impedance match (the crystal impedance lowering with increase of current) increasing energy transfer and danger of damage.

What I desire to claim and secure by Letters Patent is:

1. Apparatus for detection of radio signals in cooperation with a rectifier element including a cavity type electrical resonator of approximate axial symmetry, an axially movable end wall for tuning said resonator, an axial column in said cavity spaced from said axially movable end wall and having an outer conducting sheath connected to the end wall of said resonator opposite to the said axially movable end wall, an inner conductor within said axial column insulated from said sheath but capacitively coupled thereto by a relatively small capacitance and adapted for electrical connection to an amplifier, means for supporting one end of a rectifier element in electrical contact with said inner conductor, an aperture in said conducting sheath to avoid contacts between said sheath and said rectifier element, and means for supporting the other end of said rectifier element in contact with the outer wall of said resonator.

2. Apparatus for detection of radio signals in cooperation with a rectifier element including a coaxial cylinder type resonator of approximate axial symmetry, having an inner axial column with an outer conducting sheath connected at the base of said column to one end wall of said resonator and extending toward another end wall but at its extremity being spaced therefrom, means for adjusting the axial position of said lastmentioned end wall for tuning said resonator, an inner conducting structure within said axial column and insulated from said sheath but coupled thereto by a relatively small capacitance and comprising means for supporting and making electrical contact with one end of a rectifier element, a conducting shaft oriented axially with respect to said resonator and said column adapted for connection to an amplifier where it emerges from said resonator at the base of said column and having an annular coaxial cavity therein filled with a dielectric material and open at the end of said shaft farthest from said rectifier element, said cavity being adapted to resonate at a frequency approximately equal to the resonant frequency of said resonator and thereby to function as a choke with respect to the input of said amplifier, said conducting sheath having an aperture to avoid contact between said sheath and said rectifier element, and means for supporting the other end of said rectifier element in electrical contact with the outer wall of said resonator.

3. In a resonator adapted for cooperation with a rectifier element for detection of radio signals, said resonator having a movable end wall providing tuning thereof, a column within said resonator located axially of said resonator and protruding thereinto from one wall thereof towards but not in contact with said movable end wall, said column having an outer conducting sheath connected to the wall of said resonator at the base of said column, said column having also an inner conducting structure insulated from said sheath but capacitively coupled thereto, said inner conducting structure being adapted to support and maintain electrical contact with one end of said rectifier element which passes through an aperture in said conducting sheath, said conducting structure having an annular axial cavity filled with dielectric material and open at the end of said conducting structure which is farthest from said rectifier element supporting means, the depth and width of said cavity being such as to form a resonator of approximately the same resonant frequency as said first-named resonator, said conducting structure being adapted for electrical connection to an amplifier at its end farthest removed from said rectifier element supporting means.

4. Apparatus for detection of radio signals in cooperation with a rectifier element including a cavity resonator of the coaxial cylinder type, an axially movable end wall for tuning said cavity resonator, the inner cylindrical element of said resonator having an outer conducting sheath connected to the end wall of said cavity resonator opposite to said axially movable end wall, said element extending towards said axially movable end wall but being spaced therefrom at its extremity, an inner conducting structure within said inner cylindrical element insulated from said sheath but coupled thereto by capacitance and adapted for electrical connection to an amplifier, said inner conducting structure including means for supporting and maintaining electrical contact with one end of a rectifier element, said conducting sheath having an aperture to avoid electrical contact between said sheath and said rectifier element, means for supporting the other end of said rectifier element in contact with an outer wall of said cavity resonator, and means for capacitively coupling radio-frequency excitation to said resonator comprising at least one coaxial transmission line having a central conductor terminating in an enlargement thereof and adjustably protruding into said resonator towards the neighborhood of said extremity of said inner cylindrical element.

5. Apparatus for heterodyne detection of radio signals in cooperation with a rectifier element including a cavity resonator of the coaxial cylinder type, an axially movable end wall for tuning said cavity resonator, the inner cylindrical element of said resonator having an outer conducting sheath connected to the end wall of said cavity resonator opposite to said axially movable end wall, said element extending towards said axially movable end wall but being spaced therefrom at its extremity, an inner conducting structure within said inner cylindrical element insulated from said sheath but coupled thereto by capacitance and adapted for electrical connection to an amplifier, said inner conducting structure including means for supporting and maintaining electrical contact with one end of a rectifier element, said conducting sheath having an aperture to avoid electrical contact between said sheath and said rectifier element, means for supporting the other end of said rectifier element in contact with an outer wall of said cavity resonator, and coaxial transmission lines connecting said resonator respectively to a signal source and to a source of local oscillations, having outer conductors connected to said cavity resonator and inner conductors terminating in enlargements thereof capacitively coupled to said inner cylindrical element of said resonator in the neighborhood of the said extremity of said element.

6. Apparatus for detection of radio signals, including cavity type resonator of approximate axial symmetry, an axially movable end wall at one end of said resonator, an axial column disposed within said cavity and spaced from said movable end wall, said column having an outer conducting sheath connected to the end wall of said resonator opposite said movable end wall, said column having also an inner conductor insulated from said sheath but capacitively coupled thereto, and a rectifier element disposed radially of said cavity, one end of said element being in electrical contact with said inner conductor and the other end of said element being in electrical contact with the outer wall of said resonator.

'7. Apparatus for heterodyne detection comprising a cavity type resonator of approximate axial symmetry, an axially movable end wall at one end of said resonator, an axial column disposed within said cavity and spaced from said movable wall, said column having an outer conducting sheath connected to the end Wall of said resonator opposite said movable end wall, said column having also an inner conductor insulated from said sheath but capacitively coupled thereto and adapted for electrical connection to an amplifier, a rectifier element disposed radially of said cavity, one end of said element being in electrical contact with said inner conductor and the other end of said element being in electrical contact with the outer wall of said resonator, a source of radio signals, a source of local oscillations, and first and second coaxial transmission lines connecting said resonator respectively to said signal source and said source of local oscillations, said lines having outer conductors connected to said resonator and inner conductors terminating in enlargements thereof capacitively coupled to said column in the neighborhood of its unconnected end.

8. Apparatus for heterodyne detection comprising, a tunable cavity-type resonator, an axial column disposed within said cavity, said column having an outer conducting sheath connected to one end wall of said resonator, said column having also an inner conductor insulated from said sheath but capacitively coupled thereto and adapted for electrical connection to an amplifier, a rectifier element positioned radially of said cavity, one end of said element being in electrical contact with said inner conductor and the other end being in electrical contact with the outer wall of said resonator, a source of radio signals, a source of local oscillations, and first and second means for capacitively coupling said resonator respectively to said signal source and to said source of local oscillations.

JAMES L. LAWSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,106,771 Southworth Feb. 1, 1938 2,142,159 Southworth et al. Jan. 3, 1939 2,190,668 Llewellyn Feb. 20, 1940 2,408,420 Ginzton Oct. 1, 1946 2,433,387 Mumford Dec. 30, 1947 2,436,830 Sharpless Mar. 2, 1948 

